The present invention relates generally to evaporative emission purge systems for fuel tanks, and more particularly to gas-tightness diagnosing apparatuses for evaporative emission purge systems.
A fuel tank with an evaporative emission purge system, which purges fuel vapors evaporated in the fuel tank into an intake portion of an engine, is well known as a fuel tank mounted to an automotive vehicle. One such evaporative emission purge system equipped fuel tank has been disclosed in Japanese Patent Provisional Publication No. 6-10777 (Otsuka) published Jan. 18, 1994 (hereinafter is referred to as “JP6-10777”). An evaporative emission purge system as disclosed in JP6-10777 includes a canister, a purge control valve and an air induction valve. The canister includes an adsorbent such as an activated carbon, which adsorbs fuel vapors. The canister is mounted at some midpoint in the purge line where fuel vapors pass from the fuel tank to the intake portion of the engine. The purge control valve, which is mounted at some midpoint in a purge line between the canister and the intake portion of the engine, opens or closes the purge line. The air induction valve inducts the air (or the atmospheric pressure) into the canister during purging fuel vapors. The purge control valve and the air induction valve are electrically connected to a control unit which controls engine operations. The control unit opens or closes the purge control valve and the air induction valve depending on engine operating conditions, to save or capture temporarily fuel vapors evaporated in the fuel tank in the canister, and to purge fuel vapors into the intake portion of the engine at suitable timings.
When the purge control valve and/or the air induction valve have some failures, or the purge line is damaged, fuel vapors possibly escape into the atmosphere even in the state where the control unit stops purging the fuel vapors. In the purge system of JP6-10777, a gas-tightness diagnosing apparatus including an air pump and a pressure sensor is mounted at a purge line of fuel vapors to diagnose the gas-tightness of the purge line in order to prevent the fuel vapors leakage. In this construction, the air pump is, for example, connected at some midpoint in the purge line between the canister and the purge control valve. First in diagnosing the gas-tightness, the purge control valve and the air induction valve are closed to enclose the portion of the purge line between the fuel tank and the purge control valve. The air pump is operated in this state to send air into and to raise pressure in the enclosed line. The control unit then senses a change in the pressure in the purge line with the pressure sensor. When the pressure falls largely in a short time, the control unit determines there are some failures of escape in the purge line.